1. Field of the Invention
The present invention relates generally to fuel control systems for gas turbine engines and more specifically to those systems which effect the control of fuel flow rate through the use of electronic circuitry.
2. Description of the Prior Art
Heretofore electronic fuel controls systems for gas turbine engines have been unattractive in low cost applications where good speed performance and combustor temperature limiting are necessary or desirable. One major reason for this has been that the approach taken to controlling fuel flow has generally been to throttle the relatively constant flow of a fixed positive displacement engine mounted pump. This necessitates the use of an electrohydraulic servovalve or a motorized valve which valves may be slow acting and are generally complex and expensive. Furthermore, such approaches are not power conservative in that the fuel throttled out must be bypassed at low pressure, carrying off the heat generated by the waste fluid power.
Still other disadvantages in the use of presently available electronic fuel controls stem from the complicated electronic circuitry that has been required (ref. U.S. Pat. Nos. 2,948,114 and 2,963,860) to effect temperature limiting, acceleration override control, and minimum operational limiting during deceleration.
Further disadvantages of prior art electronic fuel control systems include the requirement of expensive and specialized sensors such as to provide an electrical signal proportional to compressor discharge pressure or to sense and provide a signal proportional to combustor temperature (see, e.g. U.S. Pat. Nos. 4,087,961 and 3,381,470). In addition to attempting to monitor the high temperature in the combustor, the use of thermocouples in this manner introduces an error signal between the temperature of the combustor and the temperature of the thermocouple which manifests itself in a time lag which must then be compensated for (ref. U.S. Pat. No. 3,893,291); i.e., the thermocouple output will lag the actual combustor temperature and would result in combustor overtemperature if left uncompensated.